Plasmas (physics)

Type of physical science: Classical physics

Field of study: Electromagnetism

Plasma is a state of matter containing positive and negative charges in nearly equal number, in which at least one set of charges is free. Plasmas occur in hot gases; permeate metals at all temperatures where negative, free electrons produce metal conduction; and fill stars so that most matter in the universe is in the plasma state.

Overview

The blazing sun, and a cold, inert metal exemplify the plasma state in its most varied form. Sometimes called the fourth state of matter, plasmas may permeate the other three states: gases, liquids, and solids. The plasma state occurs when a material has equal numbers of positive and negative electrical charges, with at least one of the charge sets free to move. Plasmas may be dense, or if large enough, they may be very sparse, but they form, by far, the main portion of the visible universe.

The plasma state seethes within the burning, dense gaseous interior of a star, such as the sun, and spreads through the sparse, outer reaches of the star's atmosphere. The dense and hot conditions within the sun, 14 million degrees Celsius at its center, free electrons from the hydrogen and helium atoms that compose the bulk of the sun and create a turbulent plasma. The hydrogen and helium atoms left behind by the freed electrons bear positive charges that neutralize the plasma. At its surface, about 6,000 degrees Celsius, the sun emits a gaseous plasma into the solar system. This sparse solar wind blows past Earth at a speed of about 400 kilometers per second, as a gentle but rapid plasma 150 million kilometers away from its source. Stellar plasmas dominate the other states of matter in the visible universe.

The violent stroke of lightning typifies the turbulent, hot environment on Earth that generates extreme plasmas. A welder channels a less violent form of lightning, held within the arc in a molten metal, to melt and cut strong metal. Electrical discharge light sources, such as in neon signs and fluorescent lamps, imprison still gentler plasmas. Even in these milder electrical discharges, the plasma temperature is surprisingly high, near 12,000 degrees Celsius for the fluorescent lamp. This plasma is twice as hot as the sun's surface.

The high temperature of many common plasmas causes a fraction of the atoms through which the plasma streams to lose electrons and become ionized. The electrons are then free to move through the material and conduct electrical current if an electric voltage is present. Power connections supply the voltage to an arc welder, neon sign, or fluorescent lamp, while clouds and the earth supply voltage for the brilliant lightning display.

The free electrons supply the negative charges in the plasma, while the neutral atoms that are stripped of an electron become positive ions that supply the neutralizing positive charges.

The light electrons move rapidly, and the heavy ions move only sluggishly so current is carried almost exclusively by electrons, if free.

In atmospheric plasmas, the free electrons may be recaptured by neutral air molecules and form negative ions that move slowly because of their heavy mass. Some plasmas, such as those found in electrolytic solutions, consist only of negative and positive ions, and the ions share electrical conduction and have conductivities greatly reduced compared with electron plasmas.

While the temperature of many gaseous plasmas is extraordinarily hot, the gas itself may be relatively cool. This event occurs if the density of the gas is below atmospheric pressure.

Such is the case in many gaseous discharge plasmas, including those in neon and decorative color signs and fluorescent lamps, where electrons dominate the plasma properties.

The large disparity between the mass of an electron and any atom allows the electron to exchange only an extremely small fraction of its energy with the heavy gas atoms when they collide within the plasma. The electrons pick up energy from the voltage applied to the plasma but cannot deliver it as easily as heat to the gas. The result is that the gas temperature, and its enclosure temperature, remains cool, while electron energy (which gives the plasma its temperature) soars. This explains the high plasma temperature of the fluorescent lamp, a temperature more than thirty times the lamp temperature. The result is, paradoxically, an energetic but cool light source.

High temperature is not necessary for all electron plasmas. In cold metals, quantum mechanics dictates that electrons are free within the body of the metal. Electrons and positive metal ions form a plasma in which the electrons move easily, but the ions remains fixed. The free and cool electrons give metals their high conductivity, while the bound metal ions give solid metals their strength. The metal, however, need not be solid; the metal, mercury, is liquid. Also, the cold solid does not need to be a metal. Modern semiconductor technology fabricates integrated circuits whose elements have minute electron plasmas that produce designed electronic functions within thin semiconductor layers sitting on isolated chips of silicon.

Scientists characterize plasmas by plasma frequency and by a plasma length. The plasma frequency is often called the Langmuir frequency, after the American chemist and the 1932 Nobel Prize winner in Chemistry Irving Langmuir (1881-1957), while the plasma length is named the Debye length after the Dutch chemist and 1936 Nobel Prize winner in Chemistry Peter Debye (1884-1966). Both scientists made pioneering contributions to the understanding of plasmas.

The free plasmas' charges oscillate at their plasma frequency when disturbed. Electrons oscillate in unison at the electron plasma frequency, and, if the ions are free to move as in gaseous plasmas, they also oscillate, but at a much lower frequency. In electrolyte plasmas, the plasma oscillates at both a negative ion and positive ion plasma frequency. The square of the plasma frequency varies directly as the charge density and inversely as the mass of the charge, hence, the lower ion plasma frequencies.

Radio, television, light, and X rays are examples, increasing in frequency, of electromagnetic waves. Electromagnetic waves are oscillations of electric and magnetic fields coupled in space and traveling at the velocity of light, about 300,000 kilometers per second, in empty space. If electromagnetic waves encounter a plasma, the electric field in the waves attempts to set up oscillations in the plasma. The plasma can respond if its plasma frequency is greater than the electromagnetic frequency, and the plasma oscillations set up will reflect the electromagnetic waves. Hence, net plasmas reflect electromagnetic waves beamed at them when the wave frequencies are below the highest plasma frequency, normally the electron plasma frequency. Higher frequency electromagnetic waves sail right through the plasmas.

Earth contains a variable, moderate density plasma--the ionosphere--that stretches from about 80 to 400 kilometers high. Its moderate charge density produces a plasma frequency above radio frequencies but well below television, light, and X-ray frequencies. The ionosphere reflects radio waves, bouncing them between Earth and itself, so that a good radio can pick up a station continents away. Television, light, and X rays go through this layer. Television leaves the curving horizon of Earth after about 80 kilometers, necessitating repeater stations. The advantage is that the light of the sun is seen through the ionosphere.

The dense electron clouds of most metals have plasma frequency above light frequency but below X-ray frequencies. Metals reflect light, and nothing can be seen through them. Metals, however, do not reflect X rays. X rays pass through thin metals and can be seen on the other side of the metals in their photographic image.

The Debye length of the plasma describes the distance that disturbances can penetrate a plasma. The square of the Debye length varies as the plasma energy and inversely as the plasma charge density, so that high-charge density plasmas have short plasma lengths. Outside disturbances, such as electric fields, are screened from the interior of the plasma at its boundaries, which have depths comparable to the plasma length. These boundaries form a sheath around the plasma, isolating the plasma from the outside. Electric fields exist within the plasma only if they generate electrical currents within the plasma, such as when the voltage is switched on to the metal wire of a hair dryer.

The boundary sheath of the plasma is very important in the plasma's operation, especially in the plasmas used for technical applications. If the plasma carries a current, the charges--normally electrons--must be able to enter and leave through distinct boundary sheaths at both ends of the plasmas. Thus, a cathode sheath forms where the electrons emit into a gaseous discharge plasma, and a different anode sheath forms where the electrons exit. Since positive ions are present in the gas, they must form in the anode sheath and become neutralized at the cathode sheath, a requirement on the sheaths that makes them quite different. All the while, the plasma must isolate itself from its containing vessel by a separate sheath to prevent loss of the current that travels the plasma. Within semiconductor devices, the boundary sheath between a metal contact and the semiconductor forms the Shottky barrier, which determines injection of current into the semiconductor. Control of sheaths between semiconductor plasmas supplies semiconductor electronics with a wide range of effects. Different effects arise when plasmas encounter magnetic fields. The free charges of a plasma carry current with ease and some electron plasmas have electrical conductivities that approach those of metals. If a metal is moved past a magnetic field, the metal generates a current that acts to expel the magnetic field from the metal. That effect produces current from the rotating metal coils in an electric generator. If plasma is blown past the poles of a magnetic field, nature delivers electricity in a variety of devices that fall under the class of magnetohydrodynamic devices.

The earth has its own magnetic field and magnetohydrodynamic devices. The solar wind plasma streaming past that magnetic field generates plasma currents that expel Earth's magnetic field from the solar wind, and, at the same time, expel the solar wind from the earth's magnetic field. The earth's magnetic field compresses on its daylight side sun and stretches out along its nighttime side. The distorted magnetic field reaches many Earth diameters into space as the magnetosphere.

Applications

The most important potential use of magnetic field compression of a plasma lies in the experimentation aimed at confining 100-million-degrees-Celsius plasmas. These experiments seek to draw energy from the sun's fusion reactions, replicated here on Earth. The magnetic fields must form a magnetic bottle that press the plasma away from the containing walls, while the plasma reaches a temperature seven times hotter than the sun's center.

The sun generates power by converting its plentiful hydrogen to helium. Normal hydrogen atoms have a single electron encircling a proton as its nucleus. In the hot plasma at the center of the sun, hydrogen atoms are stripped of their single electrons, leaving a plasma of electrons and protons. The huge weight of the overlying gases presses the protons close together and, in a series of steps, four separated protons lose mass and fuse into the nucleus of helium, two protons and two neutrons bound in a combination known as the alpha particle. The mass lost in the plasma pressure bottle converts to heat energy and works its way to the sun's surface, where it radiates into space and to Earth. On Earth, there is no pressure equivalent to that found at the sun's center. Scientists seek, instead, to capture the sun's fusion reaction within a plasma in their magnetic bottles. One design--the tokamak--bends a column of magnetic field back upon itself to form a ring. The bottled plasma sits within the hollow of the ring. The feasible gas pressures are low in the magnetic bottles and, as a result, the ions must reach temperatures well above the sun's temperature. The undertaking is awesome; the progress is painstaking, but the rewards of success are an almost endless source of energy.

By 1990, after about four decades of research, these magnetic bottle experiments had progressed to the break-even point, where the huge energy inputs equaled the energy generated in the experiments. By that time, the scale of the experiments required to confirm engineering feasibility, demanding thirty times as much energy out as in, had also grown. Scientists designed new bottles the size of commercial generator plants with outputs of 100 to 1,000 million watts for use in the twenty-first century.

There is no lack of actual applications of plasmas. Plasmas find many uses in manufacturing, such as for plasma deposition of durable coatings, of precious films, and of optical films on photographic lens. Plasmas in electrical discharges generate most of the artificial light throughout the world. Fluorescent lamps, neon signs, color display signs, and many gas lasers have plasmas that are sometimes called "glow discharge plasmas." The gas pressure within the lamp is well below atmospheric pressure and, as a result, the electron temperature is much higher than the ion and gas temperatures. Arc plasmas occur in mercury vapor lamps, sodium vapor lamps, highway lighting, and high-intensity lamps, including some that duplicate the sun's light on Earth. The gas pressure is near, or above, atmospheric pressure, and electron, ion, and gas temperatures are almost identical. Photographic flashlamps and flashlamps used to pump solid-state lasers produce high current arc plasmas that last merely a fraction of a thousandth of a second.

Context

The origins of the theoretical understanding of plasmas can be traced back to the 1870's. Plasmas are groups of positive and negative particles that interact electrically and magnetically with each other and with their surroundings. In 1873, the Scottish physicist James Clerk Maxwell (1831-1879) codified the electric and magnetic laws of nature in a set of equations that bear his name. During the 1870's, the Austrian physicist Ludwig Boltzmann (1844-1906) developed a general equation that described the way in which the constituents of a plasma change in time. In principle, the application of Maxwell's and Boltzmann's equations, although complicated at times, tells scientists almost everything needed to answer questions about any plasma.

The basic science was substantially in place well before the beginning of the twentieth century. Questions about the structure and size of plasma atoms and possible states that electrons and ions could occupy in the plasma were complicated and required experiments and a new quantum science. The English physicist Francis Hauksbee (1666-1713) must have wondered about plasma in 1706, when he produced the first man-made electrical discharge plasma. The ability of scientists to research plasma increased considerably with the fabrication of the first gaseous conduction tubes in the 1850's, by the German glassblower Johann Heinrich Wilhelm Geissler (1815-1879). Technology prompted other types of experimentation; for example, the American engineer Peter Cooper Hewitt (1861-1921) marketed the first low-pressure mercury plasma lamps in 1901, which led to the eventual introduction of fluorescent lighting in 1936. In 1960, Theodore Maiman pulsed a spiral flashlamp to pump the first ruby laser, and Ali Javan excited a helium-neon plasma with radio waves to start the first gas laser.

Langmuir and Debye made their contributions in the early part of the twentieth century.

Langmuir coined the term "plasma." With Debye's plasma length, there is now a precise answer to what constitutes plasma. Sparse plasmas have low-charge density and possess large plasma lengths and therefore wide boundary sheaths. If the boundary volume is larger than the volume of the charges, it is not considered that the charges constitute a plasma. Therefore, plasma is an assembly of opposite electric charges whose plasma length is small compared with the dimensions of the assembly or its container. The solar wind has a charge density of only a few electrons, positive ions per cubic centimeter, and a high temperature, roughly about 12,000 degrees Celsius. The plasma length is about 5 meters. In a vessel on a laboratory table top on Earth, the captured wind would not constitute a plasma; however, in the vast confines of solar space, the free wind is plasma.

Classical quantum mechanics developed in the first two decades of the twentieth century. In 1920, Indian astrophysicist Meghnad N. Saha used quantum mechanics to give plasma science the Saha equation, which relates the product of negative and positive charge densities in equilibrium at the plasma temperature. Its application is widespread in stellar plasmas, arcs, and semiconductors.

By the middle of the twentieth century, an essential understanding of formation, maintenance, stability, and disappearance of nonmagnetic plasmas was in place for selected gases. Then, magnetic confinement of high-energy plasma appeared and raised new questions without ready answers. These magnetically bottled plasmas were unstable; they sprang leaks too easily. Four decades of research plugged the leaks, giving new answers and a science of magnetic plasmas. Combined with the planetary and cosmic data available from astronomy and the space program, scientists can now understand a wide range of astronomical happenings. These phenomena include the action of the solar plasma wind on the planetary magnetic fields of the solar system, the behavior of interstellar magnetic plasma clouds within galaxies, and the brilliant emission from hot, magnetized plasma discs that surround massive black holes that, scientists believe, power distant quasars by converting mass to energy at an efficiency ten times the efficiency of solar fusion.

Principal terms

CHARGE: the quantity of electricity; atomic charges come in positive and negative multiples of the charge on the electron

ELECTRICAL CONDUCTION: the motion of electrical charges through materials

ELECTRON: the fundamental particle that carries a negative charge and is the source of electrical conduction in most materials

FLUID: a substance that flows, including liquids, gases, and free charge clouds in plasmas

ION: atoms that lose or pick up electrons form negative or positive ions; the hydrogen negative ion is a proton

Bibliography

Alfven, Hannes. "The Plasma Universe." PHYSICS TODAY 39 (September, 1986): 22-27. A short, stimulating article that focuses on plasmas. The Alfven velocity of waves in magnetized plasmas is named after the famous Swedish physicist. Through X rays, gamma rays, and radio telescopes, Alfven views a universe of plasmas whose sizes increase in three steps of 1 billion from small laboratory plasmas to magnetospheric plasmas a hundred thousand kilometers large, to interstellar clouds tens of light-years across, to the universe more than 10 billion light-years big. The rings of Saturn and giant supergalactic clouds unfold from these plasmas.

Furth, H. P. "Magnetic Confined Fusion." SCIENCE 249 (September 28, 1990): 15221528. A readable account of the state of plasma confinement for use in generating energy from the fusion reactions of the sun. The article has an interesting sense of history and a realistic assessment of the future.

Kitaigorodsky, A. I. ELECTRONS. Translated by Nicholas Weinstein. Vol. 3 of PHYSICS FOR EVERYONE, by L. D. Landau and A. I. Kitaigorodsky. Moscow: Mir, 1981. There are four small-sized books, about 250 pages each, in this set on topics in physics. They are a delight to read, and much of the material, not all, has a minimum of mathematics. Pages 65 to 81 in this volume cover gaseous discharges and plasmas.

Morse, R. L. "Plasmas." In ENCYCLOPEDIA OF PHYSICS, edited by Rita G. Lerner and George L. Trigg. Reading, Mass.: Addison-Wesley, 1981. This is a short, readable account of plasma physics, with technical details and special emphasis on plasma fusion research. Includes a few equations, however, and some technical background is needed.

Ohanian, Hans C. PHYSICS. New York: Norton, 1985. This physics text has an interesting feature. Scattered throughout the body of book are twelve "interludes" that discuss interesting topics in physics in a semitechnical fashion. Interlude J of the text has an interesting discussion of plasmas and plasma fusion.

Sunspots and Stellar Structure

Thermonuclear Reactions in Stars